At the moment they hatch, baby birds are as small as the egg they came from but by the time they leave the nest they're usually as big as their mother.
This is puzzling. How can a "baby" bird be so big? Aren't babies small by definition?
Some are. Chuck Tague captured this sweet picture of a mother wood duck at Moraine State Park. Her babies are all we expect them to be, small and very cute.
Ducks, geese and shorebirds hatch precocial young, immediately mobile, relatively mature. As soon as ducklings hatch they walk -- or in the case of wood ducks, jump -- from the nest to the lake and swim away with momma. She leads them to the safest place.
Unfortunately there is almost no safe place for a duckling. They cannot fly to escape threats and they are not very quick swimmers. Snapping turtles, water snakes and large fish take their toll. That's why ducks and geese lay up to a dozen eggs per brood. This mother wood duck is doing pretty well to have half her young still with her.
Most birds are the size of adults when we first see them. Have you noticed that you never see a baby pigeon?
Birds that nest on cliffs and don't swim -- peregrine falcons and rock pigeons, for instance -- have only one way to leave the nest. They must fly, and they must do it well enough to navigate a windy place and land safely.
For these birds the first flight is all or nothing, so the young must be adult size and fully feathered before they make the attempt. If there weren't webcams on peregrine nests we would never know the babies are small and white for several weeks. Pigeon nests don't have webcams so we never see baby pigeons.
And so I come to the surprise people registered when they saw how big our fallen juvenile peregrine was. How could he get that big in 6 weeks? Why is he so big if he only left the nest 18 days ago?
It's confusing until you remember that all baby birds grow to adult size in a matter of weeks. It's just that with baby ducks, we get to see the process.